Conventional methods such as evaporation, magnetron sputtering, and ion plating have been used for depositing metal films over integrated circuit wafers (usually made of silicon or doped silicon) for forming metal conductor patterns on top of underlying insulator films as well as between one layer of wiring and another. The films in the past have been patterned by either liftoff methods or subtractive etch methods. As the densities of metal wiring patterns have increased, the need to obtain vertical profiles on both wiring patterns as well as via patterns has emerged. One of the ways to produce such metal patterns is to pre-etch a vertical interconnecting (via) pattern or a metal wiring pattern in the underlying insulator film, followed by metal film deposition, followed by a selective removal process such as mechanical polishing, which takes metal off the high spots, leaving metal in the etched pattern (.-+.Damascene" process).
One problem with this method is that it is difficult to deposit void-free metal films when the aspect ratio (depth to width ratio) of the pattern approaches or exceeds 1.0. In order to accomplish this, there has been a requirement for highly directional deposition of the metal (normal, that is perpendicular, to the substrate plane). Evaporation can produce a metal vapor stream which is nearly collimated, but which normally has enough divergence to produce voiding by a slight shadowing at the edges. This voiding gets worse at points on a planar substrate which are away from the principal axis of the evaporator point (which is usually normal to a line from the source). Minimizing this deviation requires very large throw distances, with a corresponding decrease in deposition rate. Sputtering sources, such as magnetrons, produce a highly diffuse source of vapor which cannot coat the bottom of the pattern at a rate comparable to the top and sidewalls, so the deposit pinches off the opening.
U.S. Pat. No. 4,915,806 to Lardon, et al. illustrates how evaporation techniques may be combined with sputtering to fill high aspect ratio microcavities. An electron beam, magnetically deflected into a vaporizer crucible, provides the particle source for a particle beam. The metal particle beam source width and throw distance are selected so that the source subtends an angle of less than ten degrees when viewed from any microcavity to be filled. To prevent scattering of the beam, the process is performed under vacuum and the mean free path of the particles in the beam is maintained at a minimum of one third of the distance from source to substrate. Following initial vapor deposition, Argon gas is introduced to the chamber, and a voltage potential is established between the substrate and the chamber, causing partial sputtering to redistribute the deposited metal more evenly within the microcavity, reducing voids.
U.S. Pat. No. 4,925,542 to Kidd discusses a plasma type metal plating apparatus which increases the longitudinal velocity of moving plasma ions within a chamber in the region of the substrate, prior to incidence of those ions on the substrate. The metal ions travel in a spiral path towards the substrate, with velocity components parallel to and normal to the direction of the magnetic field. Kidd selectively enhances the longitudinal velocity component, providing a more collimated metal source than previous sputtering techniques.
Kidd uses the electron cyclotron resonance technique to generate the plasma. The metal source is located within the chamber opposite the substrate. A magnetic field is established within the chamber, and a microwave energy source is provided. The magnetic field strength and electron frequency are related by equation (1) as follows: EQU .function.=(e)(B)/[(2.pi.)(m)] (1)
where:
.function.=electron frequency PA1 B=magnetic field strength in Tesla PA1 e=electron charge PA1 m=electron mass
When the frequency of the electromagnetic energy is equal to the frequency of the electrons (resonance), then the resonant radiant energy heats the electrons. These heated electrons in turn furnish the energy to ionize the atoms of metal in the chamber, generating the plasma. The resonance zone in Kidd is located very close to the metal source.
Kidd also discusses a grounding screen interposed between the sputter source and the substrate. The grounding screen allows the substrate voltage to be increased without impacting plasma voltage, so that an electric field is established between the screen and the substrate. This electric field permits addition of energy to the plasma ions, providing the desired increase in longitudinal velocity.
Other methods such as Chemical Vapor Deposition (CVD) or Plasma Enhanced Chemical Vapor Deposition (PECVD) require higher substrate temperatures to obtain reasonable rates and low resistivities, which may be incompatible with organic insulator materials, or may be economically unattractive as a manufacturing process.